A typical gyroscope of this type is shown schematically in FIG. 1 of the accompanying drawings the gyroscope comprises a gyro rotor 10 rotatable about an axis H (and thereby producing a momentum vector along the axis). The rotor is electrically driven, power being supplied thereto by leads (not shown). The rotor 10 is mounted within a float 12. The float 12 is a sealed cylindrical vessel pivotable about its cylindrical axis O (the output axis) on stub axles 14. The axles 14 are journalled in bearings 16 mounted to a gyro case 18. The case 18 is sealed and contains a relatively dense fluid which provides both the necessary damping of float rotation and support or the mass of the float when acceleration is applied If the densities of the fluid and float are matched, i.e. neutral buoyancy, there will be no reaction forces at the output axis bearings, when acceleration is applied, and hence no friction torques caused by the bearings which torques give rise to gyro measurement inaccuracies. However, such a gyroscope may be required to operate over a temperature range in excess of 100.degree. C. (e.g. -40.degree. C. to +80.degree. C.). The density of the supporting fluid may change considerably and the reaction loads between the axles 14 and the bearings 16 may vary giving rise to changing friction torques as the gyro temperature changes. To minimise this problem, it has been suggested to excite the bearings 16 and axles 14 by low amplitude oscillation of the float 12 about the output axis O.
A floated rate integrating gyroscope was first described in, for example, GB Patent Specification No. 753449. A fuller description of this type and related modifications can be found in a publication entitled "The anatomy of a gyroscope Part II", pages 55-62, AGARD-AG313 published by AGARDOGRAPH in 1990.
It is conventional to employ miniature ball bearings on the output axis bearings in this type of gyroscope. Referring to FIGS. 2 to 5, the stub axles 14 are generally formed of tungsten carbide and the bearings 16 each comprise a case 20 preferably also made of tungsten carbide. The case 20 supports a plurality, for example seven, steel balls 21. In a typical small gyroscope, the balls 21 may each have a diameter of 0.45 mm, the diameter of the axles 14 may be 0.55 mm. At such sizes, it is conventional to provide clearance between the axle 14 and the balls of 2.5 .mu.m. This clearance is sufficient to permit the balls 21 to adopt various geometric configurations depending on the forces existing at the pivot. Consider the situation in which the gyro is operating at a temperature at which the float is not neutrally buoyant (e.g. less buoyant) in the flotation fluid. In the presence of acceleration orthogonal to the output axis O, due to gravity or vehicle manoeuvres, some reaction loads must be transmitted via the output axis bearings to support the residual `unfloated` mass of the float. FIG. 3 shows the situation in which the only force acting at the pivot 14 is that due to gravity and the balls 21 have moved, within their clearance in the case 2, to a minimum energy position. If the gyroscope is subjected to additional acceleration orthogonal to the output axis O, by lateral acceleration due to vehicle manoeuvres, then a new resultant acceleration vector will act on the axle 14, as shown in FIG. 4. The balls 21 will be displaced and will adopt a new minimum energy position in the case 20 as shown (greatly exaggerated) in FIG. 4. When the lateral acceleration ceases, there is a time interval when the situation shown in FIG. 5 pertains. The bearing arrangement there shown gives rise to a different frictional torque value between the axles 14 and the bearings 16 whereby the gyroscope bias is changed thus producing potential system errors.
It is an object of the present invention to provide an improved gyroscope control system wherein the affects of the aforesaid problem are minimised.